PETROGRAPHY— RTTYOLI TES AND TRACHYTES. 491 



not very transparent and not at all dichroitic (Plate II, Fig. 4). The pris- 

 matic sections are deeply striated, quite transparent, and have a light yellow- 

 ish-brown color. The crystals are often partially decomposed, becoming 

 quite ragged, broken and opaque and full of magnetite crystals and grains, 

 which latter form a border completely around the edge of the crystal (Plate 



I, Fig. 2). This, when carried further, changes the biotite crystal into an 

 aggregation of magnetite grains in the form of biotite, of which only an 

 occasional transparent particle remains to show the origin of the mass. 

 Biotite generally occurs in trachyte in large crystals and not in small scales 

 or leaves. 



Magnetite occurs in great abundance as grains, large masses and sharp 

 crystals. 



Minute, colorless needles of apatite have been noticed in all trachytes, 

 almost without exception, being most readily found when included in the 

 large crystals of sanidin or hornblende. It may therefore be regarded as 

 a constituent of the rock, although in minute quantities, and only discover- 

 able in the thin section under the microscope. In the Black Hills rocks, it 

 is of frequent occurrence. 



Zirkel reports tridymite to be a distinct ingredient of some trachytes, 

 in the form of small six-sided scales or leaves, arranged in groups some- 

 what as tiles or slates on a roof, but none was satisfactorily determined to 

 be present in these rocks. 



Titanite and augite are occasionally noticed, but were not found in 

 the rocks under examination. 



The groundmass, as seen under the microscope, is generally quite 

 crystalline, being usually made up of thickly crowded feldspar microlites, 

 with needles and grains of green hornblende, and magnetite grains. A 

 fluid-like structure is often recognized from the position of the hornblende 

 crystals, which appear as if imbedded in a mass flowing in streams around 

 a large sanidin crystal or grain of magnetite. There are several fine 

 examples of this structure among the trachytes [123] and [159-162] (Plate 



II, Fig. 3). The number of the feldspar microlites can only be truly appre- 

 ciated by turning the slide on its center between crossed nicols, in order to 

 cause all the crystals successively to become bright by changing their 



